1. Field of the Invention
The invention herein relates generally to a III-nitride semiconductor light emitting device that emits visible light in the amber-to-red region. A method to manufacture the same is disclosed.
2. Prior Art
Prior art III-nitride based blue light emitting structures, such as light diodes (LEDs) and laser diodes (LD), (for the sake of brevity, LEDs and LDs may each be referred to as LEDs herein) are commercially available with peak external quantum efficiency (EQE) exceeding 80%. Operating in green spectral region, the EQE of prior art LEDs drops below half that of blue LEDs. The EQE of III-nitride semiconductor light emitters, very abruptly drops even more so toward the amber and red spectra region. There are two common reasons for the efficiency loss in III-nitride light emitters: (1) a large lattice mismatch between InGaN and GaN layers of the III-nitride light emitting structure where the miscibility becomes prominent with the much higher indium concentration required for longer wavelengths; and, (2) InGaN QWs grown on c-plane polar GaN inevitably suffer from quantum-confined Stark effect (QCSE) resulting from a strong piezoelectric field, which in turn causes a reduction in the radiative recombination rate, especially in the long wavelength regions where higher indium concentration is required.
Although it is difficult to achieve InGaN-based long wavelengths (amber-to-red at wavelengths greater than 600 nm) in III-nitride light emitting devices, such as LEDs for example, such devices are very desirable in order to realize single chip, solid state lighting and monolithic multi-color light modulating devices (see U.S. Patent Application Publication Nos. 2016/0359084, 2016/0359086, 2016/0359299 and 2016/0359300). Moreover, the device performance of InGaN-based light emitting structure, such as LEDs and LDs, are less temperature dependent due to the higher bandgap offset than that of other long wavelength light emitting structures such as light emitters based on an AlInGaP material system. In addition, a GaN-based red wavelength emitting LED material structure is beneficially temperature-expansion matched to GaN-based blue and green LEDs, which makes it compatible with GaN-based stacked LED light emitting structures that use wafer bonding to create multi-color solid state light emitters (see U.S. Pat. Nos. 7,623,560, 7,767,479 and 7,829,902). Thus, InGaN-based long wavelength light emitting structures, such as LEDs and LDs, can be superior in many applications.
Within the field of prior art InGaN-based red wavelength light emitters, such as LEDs or LDs, that are grown along the crystalline c-axis, all exhibit “phase separation” (also known to a person skilled in the art as indium segregation) due to poor material quality, see for example R. Zhang et al. in U.S. Pat. App. Publ. 20110237011A1 entitled “Method for forming a GaN-based quantum well LED with red light” and Jong-II Hwang et al in App. Phys. Express 7, 071003 (2015) entitled “Development of InGaN-based red LED grown on (0001) polar surface”. This phase separation manifests itself as one or more extra emission peaks in shorter wavelength regions on the spectra, which inevitably reduces color purity as shown in FIGS. 2(b) and (c). Therefore, approaches for increasing indium incorporation while not compromising material quality and device performance are critical to achieve long wavelength emission, amber-to-red, III-nitride based light emitting structures, such as LEDs and LDs. The methods and devices disclosed herein pave the way for high performance, long wavelength III-nitride semiconductor light emitting devices for use in solid state lighting, display systems and many other applications that require greater than 600 nm wavelength solid state light emitters.